Metgasco groundwater model

A subregional groundwater model was developed by Parsons Brinckerhoff, which was commissioned by Metgasco (Parsons Brinckerhoff, 2013). The model aids the design and optimisation of a monitoring network within Metgasco’s Petroleum Exploration Licences (PEL 13 and PEL 16) by providing preliminary estimates of drawdown resulting from the proposed coal seam gas (CSG) development scheme. The model covers an area of 1250 km2 west of Casino (Figure 7) and was designed as a ‘Class 1’ model in terms of confidence levels in accordance with the Australian groundwater modelling guidelines (Barnett et al., 2012). Three development pathways were modelled: firstly, a baseline model; secondly, a model with a conductive fault; and thirdly, a model with a low-permeable fault. The Drain package of MODFLOW (Harbaugh et al., 2000) was used to simulate 90 CSG wells based on the proposed development plan.

Eleven layers were used to represent nine hydrostratigraphic units, where the Maclean Sandstone Member was divided into three layers and the Walloon Coal Measures constituted the bottom layer. The model was discretised into 182 columns and 389 rows with a grid spacing that varies from 100 m within the proposed petroleum production lease (PPL) to 225 m outside it. General-head boundaries were assigned to the western and eastern sides of the model domain. The MODFLOW Drain package was used to simulate rivers and streams. Zones were used to distinguish different outcropping hydrogeological units in the top layer. Hydraulic conductivity of the confined bedrock formations was estimated from a depth-dependent relationship derived from data obtained from other geologically similar sedimentary basins. A constant storativity coefficient of 10-5 was used across the model domain, and a specific yield of 0.1 for the unconfined layers. Recharge was assumed to be 4% of the mean annual rainfall recorded at the meteorology station at Casino Airport. Potential evapotranspiration was implemented using the MODFLOW EVT package. The potential evapotranspiration rate was assumed to be 50% of the long-term mean annual pan evaporation of 1535 mm/year at Alstonville Tropical Fruit Research Station (058131) that is 45 km to the east of the model domain and the closest available evaporation data for the model area. The assumed potential evapotranspiration rate decreases gradually to an extinction depth of 4 m. The model contains a steady-state stress period and a transient stage of 51 years that were divided into 21 stress periods.

The preliminary modelling results indicate that pressure drawdown in the target coal seams can propagate into the overlying aquifers and aquitards to a maximum of up to 5 m. Although the maximum drawdown exceeds the 2 m threshold set by the NSW aquifer interference policy (NSW DPI, 2012), the 2 m impact zone is only restricted within the PPL. The drawdown impact on the watertable was found to be less than the seasonal variations in watertable levels within the modelled area. The modelling results also indicate that a fault intersecting all the geological units has an insignificant impact on the predictions of drawdown. The assignment of a high hydraulic conductivity to the fault results in a minor drawdown increase of less than 2 m in the Kangaroo Creek Sandstone in the close vicinity of the fault. On the other hand, a fault with a low hydraulic conductivity suppresses the propagation of drawdown and tends to restrict the impact within faults blocks. The additional drawdown due to CSG extraction is unlikely to be measurable at the watertable. No formal uncertainty analysis was performed for all the predictions, although a very preliminary sensitivity analysis was carried out by altering the hydraulic conductivities of some aquifers.

Despite the existence of the three models mentioned above, none of them are deemed to be suitable for groundwater modelling for the current bioregional assessment (BA). The spatial domains covered by the Arrow model and the Alstonville model are distant from the location where the additional coal resource development, West Casino Gas Project, is likely to occur. The Metgasco model lacks some advanced features required by BA, such as transient boundaries and very simplified parameterisation, although it was developed for the West Casino Gas Project. Additionally, the Metgasco model did not simulate the potential development in the gas field south of Casino corresponding to a petroleum production lease application (PPLA9). The confidential status of this model also precludes its usage in this BA.

Last updated:
23 October 2018
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